Here's what I did on my '89 Taurus. System still had a bit of R12 in it so I added a couple of cans of ESTER lubricant and filled it with 134A until the air coming out of the vents was about 36 degrees. It worked just fine for three years but now I have to add some more 134A because it doesn't get very cold, there's a slow leak. Total cost was less than $40.
If you are going to pay someone to do it maybe keeping it r12 is okay, because shops don't want to work on "bastardized" systems. But in my case I figured it was worth the risk and it really paid off, especially with the law being if the R12 leaked out the leak HAS TO BE FIXED before it is recharged by a repair shop and that means buying maybe three pounds at $100 a pound. Even if they pumped two pounds out!

R.